Synchronous digital systems employ clock signals to coordinate the transmission and receipt of data. For example, a transmitter might synchronize transmitted data to a clock signal and then convey the synchronized data and clock signals to a receiver. The receiver might then recover the data using the clock signal. High-performance digital transmitters often communicate data unaccompanied by a clock signal with which to synchronize the receiver. Instead, the receiver phase-aligns a locally generated receive clock signal to the incoming data and uses the phase-adjusted “recovered” clock signal to sample the data. Receive circuitry for sampling data using a recovered clock signal is commonly referred to as “clock and data recovery” (CDR) circuitry.
High-performance communication channels suffer from many effects that degrade signals. Primary among them is inter-symbol interference (ISI) from high frequency signal attenuation and reflections due to impedance discontinuities. ISI becomes more pronounced at higher signaling rates, ultimately degrading signal quality such that distinctions between originally transmitted signal levels may be lost. Some receivers therefore mitigate the effects of ISI using one or more equalizers, and thus increase the available signaling rate. Typical types of equalizers include linear equalizers, feed-forward equalizers (FFEs), and decision-feedback equalizer (DFEs).